In dozens of pages of dispassionate legal prose, the methods approved by the Bush administration for extracting information from senior operatives of Al Qaeda are spelled out in careful detail — like keeping detainees awake for up to 11 straight days, placing them in a dark, cramped box or putting insects into the box to exploit their fears. The interrogation methods were authorized beginning in 2002, and some were used as late as 2005 in the C.I.A.’s secret overseas prisons. The techniques were among the Bush administration’s most closely guarded secrets, and the documents released Thursday afternoon were the most comprehensive public accounting to date of the program.

Mr. Obama condemned what he called a “dark and painful chapter in our history” and said that the interrogation techniques would never be used again. But he also repeated his opposition to a lengthy inquiry into the program, saying that “nothing will be gained by spending our time and energy laying blame for the past.”

Some senior Obama administration officials, including Attorney General Eric H. Holder Jr., have labeled one of the 14 approved techniques, waterboarding, illegal torture. The United States prosecuted some Japanese interrogators at war crimes trials after World War II for waterboarding and other methods detailed in the memos.

The release of the documents came after a bitter debate that divided the Obama administration, with the C.I.A. opposing the Justice Department’s proposal to air the details of the agency’s long-secret program. Fueling the urgency of the discussion was Thursday’s court deadline in a lawsuit filed by the American Civil Liberties Union, which had sued the government for the release of the Justice Department memos.

Together, the four memos give an extraordinarily detailed account of the C.I.A.’s methods and the Justice Department’s long struggle, in the face of graphic descriptions of brutal tactics, to square them with international and domestic law. Passages describing forced nudity, the slamming of detainees into walls, prolonged sleep deprivation and the dousing of detainees with water as cold as 41 degrees alternate with elaborate legal arguments concerning the international Convention Against Torture.

The documents were released with minimal redactions, indicating that President Obama sided against current and former C.I.A. officials who for weeks had pressed the White House to withhold details about specific interrogation techniques. Leon E. Panetta, the C.I.A. director, had argued that revealing such information set a dangerous precedent for future disclosures of intelligence sources and methods.

A more pressing concern for the C.I.A. is that the revelations may give new momentum to proposals for a full-blown investigation into Bush administration counterterrorism programs and possible torture prosecutions.

Within minutes of the release of the memos, Senator Patrick J. Leahy, the Vermont Democrat who is chairman of the Senate Judiciary Committee, said that the memos illustrated the need for his proposed independent commission of inquiry, which would offer immunity in return for candid testimony.

Mr. Obama condemned what he called a “dark and painful chapter in our history” and said that the interrogation techniques would never be used again. But he also repeated his opposition to a lengthy inquiry into the program, saying that “nothing will be gained by spending our time and energy laying blame for the past.”

Mr. Obama said that C.I.A. officers who were acting on the Justice Department’s legal advice would not be prosecuted, but he left open the possibility that anyone who acted without legal authorization could still face criminal penalties. He did not address whether lawyers who authorized the use of the interrogation techniques should face some kind of penalty.

The four legal opinions, released in a Freedom of Information Act lawsuit filed by the A.C.L.U., were written in 2002 and 2005 by the Justice Department’s Office of Legal Counsel, the highest authority in interpreting the law in the executive branch.

The first of the memos, from August 2002, was signed by Jay S. Bybee, who oversaw the Office of Legal Counsel, and gave the C.I.A. its first detailed legal approval for waterboarding and other harsh treatment. Three others, signed by Steven G. Bradbury, sought to reassure the agency in May 2005 that its methods were still legal, even when multiple methods were used in combination, and despite the prohibition in international law against “cruel, inhuman or degrading” treatment.

All legal opinions on interrogation were revoked by Mr. Obama on his second day in office, when he also outlawed harsh interrogations and ordered the C.I.A.’s secret prisons closed.

In the memos, the Justice Department authors emphasized precautions the C.I.A. proposed to take, including monitoring by medical personnel, and the urgency of getting information to stop terrorist attacks. They recounted the C.I.A.’s assertions of the effectiveness of the techniques but noted that interrogators could not always tell a prisoner who was withholding information from one who had no more information to offer.

The memos include what in effect are lengthy excerpts from the agency’s interrogation manual, laying out with precision how each method was to be used. Waterboarding, for example, involved strapping a prisoner to a gurney inclined at an angle of “10 to 15 degrees” and pouring water over a cloth covering his nose and mouth “from a height of approximately 6 to 18 inches” for no more than 40 seconds at a time.

But a footnote to a 2005 memo made it clear that the rules were not always followed. Waterboarding was used “with far greater frequency than initially indicated” and with “large volumes of water” rather than the small quantities in the rules, one memo says, citing a 2004 report by the C.I.A.’s inspector general.

Most of the methods have been previously described in news accounts and in a 2006 report of the International Committee of the Red Cross, which interviewed 14 detainees. But one previously unknown tactic the C.I.A. proposed — but never used — against Abu Zubaydah, a terrorist operative, involved exploiting what was thought to be his fear of insects.

“As we understand it, you plan to inform Zubaydah that you are going to place a stinging insect into the box, but you will actually place a harmless insect in the box, such as a caterpillar,” one memo says.

Mr. Bybee, Mr. Bradbury and John Yoo, who was the leading author of the 2002 interrogation memos, are the subjects of an investigation by the Justice Department’s ethics office about their legal analysis on interrogation. Officials have described the draft ethics report, by the Office of Professional Responsibility, as highly critical, but its completion has been delayed to allow the subjects a chance to respond.

The A.C.L.U. said the memos clearly describe criminal conduct and underscore the need to appoint a special prosecutor to investigate who authorized and carried out torture.

But Dennis C. Blair, the director of national intelligence, cautioned that the memos were written at a time when C.I.A. officers were frantically working to prevent a repeat of the Sept. 11, 2001, attacks.

“Those methods, read on a bright, sunny, safe day in April 2009, appear graphic and disturbing,” said Mr. Blair in a written statement. “But we will absolutely defend those who relied on these memos.”